1. Field of the Invention
The present invention relates to an immersion exposure apparatus for exposing a substrate through a liquid and a device manufacturing method using the same.
2. Description of the Related Art
A projection exposure apparatus which exposes a substrate such as a wafer with a circuit pattern drawn on an original such as a reticle (mask) by a projection optical system has conventionally been used. Recently, there is an increasing demand for the exposure apparatus of high resolution and high quality. Under such a circumstance, attention is being directed to an immersion exposure method as one of ways for responding to such a demand for high resolution (U.S. Pat. No. 5,121,256). The immersion exposure method serves to increase further the numerical aperture (NA) of the projection optical system by placing a liquid as the medium at the wafer side of the projection optical system. When the refractive index of the medium is n, NA=n·sin θ is established. Accordingly, the NA of the projection optical system can be increased up to n if a part of the apparatus is filled with a medium of refractive index (n>1), which is higher than the refractive index of air. As a result, the resolution R (R=k1(λ/NA)) of the exposure apparatus represented by a process constant k1 and a wavelength λ of a light source is raised.
A local fill type immersion exposure apparatus in which a space between a final lens of the projection optical system and a portion of a surface of the wafer is partially filled with liquid has been discussed in International Publication No. WO99/49504. In the local fill type immersion exposure apparatus, a wafer is exposed while being moved with respect to the projection optical system. Accordingly, bubbles or turbulent flow may be generated in the liquid between the projection optical system and the wafer. The bubbles shield exposing light and the turbulent flow applies a pressure to the final lens of the projection optical system to cause a minute deformation of the final lens, which results in an occurrence of aberration in the projection optical system. Therefore, in order to prevent a transfer property of the immersion exposure apparatus from deteriorating, it has been proposed that a portion of the projection optical system contacting the liquid is subjected to a surface treatment so as to allow the portion of the projection optical system contacting the liquid to have compatibility with the liquid.
Further, Japanese Patent Application Laid-Open No. 2004-207696 and Japanese Patent Application Laid-Open No. 2004-207710 discuss a liquid support plate designed to be substantially flush with the wafer and to be arranged around the wafer so that the liquid will not spill when a shot (edge shot) of a wafer end is exposed to light in the local fill type immersion exposure apparatus.
However, in the local fill type immersion exposure apparatus, there is a problem that a droplet may remain on the liquid support plate when the wafer is exposed to light while it is moved together with the liquid support plate. The remaining droplet may oxidize the liquid support plate which results in contamination of the wafer. Further, the droplet remaining on the liquid support plate may fly apart while the wafer and the liquid support plate are moved and cause contamination or corrosion of components around the residual droplet. Whether the liquid in the form of the droplet remains on the liquid support plate depends on the contact angle of the surface of the liquid support plate.
The above-described local fill type immersion exposure apparatus has the following problems.
If ultraviolet rays (exposure light) having a high energy such as a KrF laser or an ArF laser fall onto the surface of the liquid support plate while an immersion liquid (for example, purified water) attaches to the surface of the liquid support plate, surface conditions of ceramics such as SiC and Teflon® used as materials of the liquid support plate will change. Generally, the contact angle of the ceramics will be remarkably decreased. For example, in the case of SiC, the contact angle decreases from about 50 degrees before exposure to about 10 degrees after irradiation of the exposure light. Further, the surface of the resin will be shaved as the contact angle decreases.
The following problem will occur if the contact angle of the surface of the liquid support plate increases (i.e., if the liquid repellency lowers) . If the contact angle of the liquid with respect to the surface of the liquid support plate is smaller than that of a resist (photosensitive resin) applied on the wafer, liquid may remain on the liquid support plate in the form of a droplet when the liquid film moves from the liquid support plate to the surface of the wafer. If the droplet remains on the liquid support plate, it may invite wafer contamination. Further, the residual droplet may fly apart while a stage is moving, which causes contamination or corrosion of components around the stage.